The effects of a rehabilitation programme one year after myocardial infarction (MI) were investigated in 171 patients under 65 years of age. These patients were allocated at random to rehabilitation and control groups before discharge from hospital. The groups were comparable with regard to age, sex and clinical data. The programme included physical exercise, counselling of patients and relatives, and social measures over a 3-month period during the convalescent stage. One year after MI patients in the rehabilitation group showed lower systolic blood pressure at rest and lower diastolic pressure on submaximal exercise than controls. No differences were found with regard to mean work capacity, days off work, return to work, psychological status, and understanding of the illness. At 12 months all patients were less physically and socially active than before MI, they were more dependent on their relatives than before, and they had poor understanding of their illness.